A hospital doctor whose wife and four children were killed in a suspicious house fire "fought hard to save his family in appalling conditions", police said.

Abdul Shakoor lost his wife Sabah Usmani, 44, who was also a doctor, their sons Sohaib, 11, Muneeb, nine and Rayan, six, and daughter Hira, 12, in the blaze at their end terrace home in Barn Mead, Harlow. Another daughter, Maheen, three, is in a critical condition in Chelmsford's Broomfield hospital, where she is being treated for serious burns.

Police said Shakoor, who was inside the property when the fire started and is believed to have jumped to safety, was being treated for severe smoke inhalation in intensive care at Harlow's Princess Alexandra hospital, where he works. His condition was described as stable.

He was being supported by family liaison officers and was in severe shock, said Gary Beautridge, assistant chief constable of Essex police, who is in charge of the investigation.

Police are waiting to speak in detail to Shakoor to establish the facts. "But, inevitably this will take time because of the emotional trauma he has suffered," Beautridge said. He said it was an isolated incident and police believed "the answers to this inquiry could lie within the local community of Harlow".

Emergency services were called to the house at 1.48am on Monday and found the property well ablaze. Shakoor was outside the premises. Firefighters recovered his wife and children, and Muneeb and Maheen were still alive, but the boy died in hospital.

Forensic teams were at the scene and were also examining a Ford Focus car that was set alight in the same road some distance from the house. The car belonged to another resident in the street, and was not connected to the family.

Police said one line of inquiry was to establish whether the car fire was coincidence. "The reality is we do not yet know what has caused that fire or the fire in the vehicle," Beautridge said.

The family, originally from Karachi, moved from Pakistan to the UK in 2009, and to Harlow last year. They were said to be well known at Friday prayers, and the children attended local schools.

Sabah Usmani with her children. Photograph: Guardian screengrab from Facebook

Police said there was no information to suggest the fire was connected to the local ethnic minority community. They appealed for witnesses to come forward. Police patrols in Harlow have been stepped up.

Friends and neighbours paid tribute to a "beautiful, well-behaved family". Safia Anwar, 38, said: "The children were best friends with our own. We would see them at mosque and at the school gates. They were just such a lovely family. I last saw Sabah on Friday. She seemed normal with no worries at all."

The chief fire officer David Johnson said the blaze started downstairs while the family were asleep upstairs and developed rapidly, suggesting some kind of fuel may have been used.

The incident commander, Steve Foster, of Essex fire and rescue, said: "Crews arriving at the scene did everything they could. They were faced with a rapidly developed fire and went straight into the burning building wearing breathing apparatus to bring people out as quickly as possible. Once casualties were out, they worked alongside ambulance teams to do all they could."

Neighbour Sharon Pavey said: "I saw flames coming out of the back window and dialled 999 but they were already on their way. The flames and smoke were billowing out and we could tell it was very serious. I don't think many people knew them but we are all in shock and this is very sad news."

• This article was amended on 16 October 2012 to correct the spelling of Abdul Shakoor